Prior to the present invention as shown by Cizek U.S. Pat. No. 3,383,435, assigned to the same assignee as the present invention and incorporated herein by reference, thermoplastic resin compositions were provided comprising a polyphenylene ether and a styrene resin. Other thermoplastic compositions are shown by Izawa et al., U.S. Pat. Nos. 3,929,930 and 3,929,931, incorporated herein by reference, which are polyphenylene ether having polystyrene grafted onto the backbone. Although these materials in the form of shaped articles have high impact strength, stiffness, good surface appearance, heat resistance and other desirable properties during or after molding, a major shortcoming of these materials as thermoplastics resins is their normally flammable nature. As a result, flame retardant and/or drip retardant agents are commonly incorporated into such blends of polyphenylene ethers and styrene resin prior to molding.
The flammability of normally flammable thermoplastic polymers have been reduced by using antimony-, halogen-, phosphoous- or nitrogen-containing additives commonly referred to as flame retardant agents. For example, aromatic phosphates such as triphenylphosphate or a combination of such compounds with other compounds such as halogenated aromatics have been added as flame retardant agents as shown by Haas, U.S. Pat. No. 3,639,506.
As shown in copending application 651,542 filed Sept. 17, 1984, significant improvements with respect to imparting improved flame retardant properties to polyphenylene oxide-polystyrene blends have been achieved by utilizing an effective amount of an organothiophosphate as a flame retardant in such polyphenylene oxide-polystyrene blends. It has been found, however, that while the employment of minor amounts of organothiophosphates in polyphenylene oxide-polystyrene blends have imparted improved flame retardant properties to such blends without adversely affecting the heat distortion property of such molded materials, the resulting molded products often have an undesirable sulfur odor.
The present invention is based on the discovery that flame retardant polyphenylene oxide-polystyrene blends can be made in accordance with the practice of the present invention by utilizing a minor amount of an organothiophosphate in combination with an effective amount of zinc oxide which has been found to improve the flame retardant performance of such polyphenylene oxide-polystyrene blends without adversely affecting the heat distortion temperature of such molded blends while resulting in the production of molded parts which are substantially free of undesirable sulfur odor.